In satellite communication systems, a transmitting earth station generates a modulated carrier in the form of electromagnetic waves up to a satellite, forming an "uplink". The incident electromagnetic waves are collected by the satellite, processed electronically to reformat the modulated carrier in some way, and re-transmitted to receiving earth stations, forming a "downlink". The earth stations in such communication systems basically consist of a transmitting and/or receiving power station functioning in conjunction with an antenna subsystem and form strategic parts of the satellite communication system.
A TVRO earth station typically comprises a receiving antenna such as a paraboloidal dish, a low noise block (LNB) converter (or a low noise amplifier (LNA) and a down converter) located at an outdoor antenna site, and a receiver located near an indoor television set. The down converter and the receiver are usually connected by a coaxial cable.
One of the problems encountered in TVRO earth stations in certain parts of the country is terrestrial interference, which is commonly referred to as "TI". TI is caused by the presence of local terrestrial microwave communication links, and can cause substantial degradation of the signals received by TVRO earth stations from orbiting satellites. One of the most common manifestations of TI in an actual television picture is the appearance of either black or white "sparklies". If the TI is on one side of the center frequency of the transponder to which the receiver is tuned, the "sparklies" are white, and if the TI is on the other side of the center frequency, the "sparklies" are black.
TI can also lead to other problems in a TVRO earth station receiver. For example, if the receiver has AFC, it can attempt to lock onto the TI rather than the desired signals from a selected satellite transponder; although TI is more narrow-banded than the satellite signals, the TI often has a greater signal strength than the satellite signals. Furthermore, most terrestrial microwave signals are dithered, using a 30-Hz energy dispersal waveform, which can cause TI to move in and out of the bandwidth of the desired satellite signals, or back and forth between different portions of the bandwidth of the satellite signals.
Other problems that arise in TVRO earth stations can be caused by downconverter drift or the asymmetry of the satellite video signals. Commercially available downconverters can drift by as much as 4 MHz due to seasonal temperature variations, because these downconverters are normally located at the outdoor antenna site. This drift shifts the center frequencies of the various satellite channels away from the nominal center frequencies where they are expected to be located. A similar problem is presented by the asymmetry of the frequency-modulated satellite video signals. Unlike amplitude-modulated video signals, the signal spectra of the satellite video signals are often (but not always) asymmetrical, which causes the centers of the signal spectra to be offset from the respective nominal center frequencies.